Charged-particle multi-beam processing apparatuses where a method of the mentioned type is used are well-known in prior art. In particular, the applicant has realized charged-particle multi-beam devices as described in several patents in the name of the applicant with respect to the charged-particle optics, pattern definition (PD) device, and multi-beam writing methods employed therein. For instance, a 50 keV electron multi-beam writer which allows to realize leading-edge complex photomasks for 193 nm immersion lithography, of masks for EUV lithography and of templates (1× masks) for imprint lithography, has been implemented, called eMET (electron Mask Exposure Tool) or MBMW (multi-beam mask writer), for exposing 6″ mask blank substrates. Moreover, a multi-beam system also referred to as PML2 (Projection Mask-Less Lithography) was implemented for electron beam direct write (EBDW) applications on Silicon wafer substrates. The multi-beam processing apparatuses of the said kind are hereinafter referred to as multi-beam writer, or short MBW.
In the case of a MBW, the scanning stripe exposure is of the type where a structured beam composed of a plurality of beamlets is directed onto the target and moved along a path over the exposure region wherein between subsequent exposure steps the structured beam is shifted on the target by consecutive distances corresponding to an exposure length which is smaller than the width of the structured beam on the target.
As a typical implementation of a MBW, the applicant has realized a 50 keV electron writer tool, which implemented a total beam size of 20 nm comprising 512×512 (=262,144) programmable beamlets within a beam array field of dimensions 81.92 μm×81.92 μm at the substrate. In this writer tool a typical type of substrate is a 6″ mask blank (having an area of 6″×6″=152.4 mm×152.4 mm and thickness of e.g. 1″/4=6.35 mm) covered with an electron beam sensitive resist; furthermore, multi-beam writing is possible on resist-covered 150 mm Si wafers as well. Further information about this writer tool of the MBW type can be found in U.S. Pat. No. 9,653,263 of the applicant, which is herewith incorporated into this disclosure by reference. The MBW is configured to perform a writing method which herein is referred to as “scanning stripe exposure”. The scanning stripe exposure writing method is discussed below inasmuch as needed in the context of the invention with reference to FIGS. 1 to 7; further details about scanning stripe exposure can be found in in U.S. Pat. No. 9,053,906 of the applicant, which is herewith incorporated into this disclosure by reference.
Another state-of-the-art writer technology which is used to expose a pattern, such as a mask pattern on a glass substrate, is the so-called VSB technology (variable shaped beam). The VSB technology is based on a sequential delivery of “shots” on the substrate whereas the shots are adjustable in size and the dose per shot is controllable by a high-speed blanker. Typically, the current density of an advanced VSB writer is very high (100-1000 A/cm2), whereas in a MBW the current density is in the order of 1-4 A/cm2. Thus, the VSB writer current density is higher by a factor of up to 1000 as compared to a MBW. The improved productivity of a MBW originates from the very large number of beams (“beamlets”), which is typically in the order of 250 thousand or more. Hence, a multi-beam writer can theoretically deliver up to 250 times higher current than a VSB writer, despite the lower current density, if the same beam size is applied. Considering all instrumental and physical limitations such as Coulomb interaction within the particles in the beams, the multi-beam writer can practically still deliver about 10-25 times more current than a VSB writer, which explains the improvement in productivity.
In the design of patterns for exposure in charged particle writers, such as a MBW or VSB writer, it is common to assign a certain exposure dose level to the features; this exposure dose level is herein referred to as “assigned dose”. It is common to use the double of the dose-to-clear (where “dose-to-clear” is herein used to denote the dose that just suffices to achieve positive exposure, i.e., development of the resist, and is abbreviated as DDtC) as a standard value for the assigned dose; however, for certain cases, such as certain approaches for correction of feature sizes, the assigned dose may be modified to a different value. Usually, the assigned dose is raised—so-called “overdosing” (or “underdosing”, where the dose of the relevant feature is reduced). While from the viewpoint of lithography, overdosing (or underdosing) of features has little to no impact on the quality of the exposure process, the state-of-the-art industrial user is used to VSB-based techniques where it is common to work with significant dose adjustments to correct for processing-related sizing effects such as by etching/erosion or pattern density related loading effects, wherein the specific amount of overdosing corresponds to the desired contraction or expansion of feature size, respectively. This may result in patterns where different pattern components have widely varying exposure dose levels, and in extreme cases, such dose adjustments can range from −40% underdosing up to +300% overdosing or more.
The assigned dose D of a feature is often expressed as the so-called dose factor, which is the assigned dose scaled to the double of the dose-to-clear (D/2DDtC). This reflects the notion that a dose factor of 1 realizes an assigned dose which is the double of the dose-to-clear, realizing what is called “isofocal dose” since a change of focus (or, similarly, change of blur) will have a minimal impact on features written at or near the “isofocal dose”.